<!DOCTYPE html>
<html>
  <head>
    <title>Test mpadded</title>
  </head>
  <body>
    <math>
      <!--height in term of height should not depend on the characters-->
      <mpadded mathbackground="red" height="100height" depth="0">
        <mphantom>
          <mtext style="font-family: monospace">_</mtext>
        </mphantom>
      </mpadded>
    </math>
    <math>
      <mpadded mathbackground="red" height="100height" depth="0">
        <mphantom>
          <mtext mathvariant="monospace">i</mtext>
        </mphantom>
      </mpadded>
    </math>
  </body>
</html>
